Pipe structure of branch pipe line

ABSTRACT

A water pipe structure of a branch pipe line ( 12 ) connected to a main pipe ( 22 ) for flowing water therethrough, wherein a cavity flow suppressing structure ( 30 ) is installed between the main pipe ( 22 ) and the branch pipe line ( 12 ) or in the branch pipe line to suppress a cavity flow produced in the closed branch pipeline, whereby the adverse effect of a thermal stratification formed by the cavity flow on the pipe can be eliminated.

FIELD OF THE INVENTION

The invention relates to a water piping arrangement, and in particularto a water piping arrangement for avoiding adverse effect to the pipingsystem based on a thermal stratification which is formed by a cavityflow which is generated in a closed pipe branched from a main pipe

DESCRIPTION OF THE PRIOR ART

A various branch pipes are generally connected to a main water pipe in apower plant or the other types of plant, in which some branch pipes areused during only the starting operation or the maintenance of the plantand closed by a shut-off valve provided in the branch pipes after theoperation of the plant is transferred to a normal operation.

The inventors of the present application have found that such a branchpipe closed by a shut-off valve (hereinafter, referred to a closedbranch pipe) functions as a deep recess formed in a main pipe, and that,within the closed branch pipe, a cavity flow is induced by the waterflow in the main pipe. If the cavity flow is affected by a heatdissipating action of the wall of the branched pipe, a thermalstratification appears in the water within the closed branch pipe, andthe water temperature is suddenly changed across the thermalstratification so that a large thermal stress is generated in the pipe.

In the prior art, such a thermal stress in the pipes, based on thethermal stratification generated by the cavity flow, is not consideredin the calculation of the piping design. If a thermal stratificationappears in a pipe, in particular in an elbow joint of the piping system,a crack may be resulted in the elbow joint due to the thermal stress inthe elbow joint.

DISCLOSURE OF THE INVENTION

The invention is directed to solve the above-described problem of theprior art, and to provide a piping arrangement of a branch pipe foravoiding adverse effect to the piping system, based on a thermalstratification generated by a cavity flow in the closed branch pipe.

According to the invention, there is provided a piping arrangement whichcomprises a main pipe allowing a water flow; a branch pipe connected tothe main pipe; and cavity flow inhibiting means provided between themain pipe and the branch pipe.

The cavity flow inhibiting means may comprise:

a swirl preventing plate including at least two plates which intersecteach other with an intersecting line extending in the direction of theflow in the branch pipe;

a sleeve which has a inner diameter larger than the outer diameter ofthe branch pipe and enclose a portion of the branch pipe connected tothe main pipe;

a deflecting member provided over a portion of the branch pipe connectedto the main pipe;

an orifice provided in the branch pipe;

a tube member which is provided in a portion of the branch pipeconnected to the main pipe and has different inner diameters one ofwhich is larger than that of the closed branch pipe;

an entrance radius enlarged portion with the sectional area of its flowchannel being gradually reduced from the main pipe toward the branchpipe. cross pipe; or

an inclined connecting pipe for obliquely connecting the branch pipe tothe main pipe.

According to another feature of the invention, a piping arrangement of abranch pipe connected to a main pipe for allowing water to flowtherethrough is provided. The branch pipe includes a cross pipeconnected perpendicularly to the main pipe and a horizontal pipeconnected to the cross pipe by an elbow joint so as to horizontallyextend. The piping arrangement is characterized by the elbow joint beingdisposed in an area nearer than a transition ozone where a cellularvortex, generated in the branch pipe, is transformed into twistervortex.

The elbow joint is preferably disposed in a range within six times ofthe inner diameter of the branch pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic illustration of a laboratory equipment showingthe formation of a cavity flow generated in a closed branch pipe;

FIG. 1B is a section along line b—b in FIG. 1A;

FIG. 2 is the laboratory equipment showing from another direction;

FIG. 3 is a schematic illustration of the laboratory equipment showingits configuration;

FIG. 4A is a section of a piping arrangement similar to FIG. 1A, showingan example of the cavity flow inhibiting means;

FIG. 4B is a section along line VI—Vi in FIG. 4A,

FIG. 5 is a section of a piping arrangement similar to FIG. 4A, showinganother embodiment of the cavity flow inhibiting means;

FIG. 6 is a section of a piping arrangement similar to FIG. 4A, showinganother embodiment of the cavity flow inhibiting means;

FIG. 7A is a section of a piping arrangement similar to FIG. 4A, showinganother embodiment of the cavity flow inhibiting means;

FIG. 7B is a perspective view showing an example of an orifice as thecavity flow inhibiting means;

FIG. 7C is a perspective view showing another example of an orifice asthe cavity flow inhibiting means;

FIG. 8 is a section of a piping arrangement similar to FIG. 4A, showinganother embodiment of the cavity flow inhibiting means;

FIG. 9A is a section of a piping arrangement similar to FIG. 4A, showingan entrance radius enlarged portion as the cavity flow inhibiting means;

FIG. 9B is a section of a piping arrangement similar to FIG. 4A, showinganother example of the entrance radius enlarged portion as the cavityflow inhibiting means;

FIG. 10 is a section of a piping arrangement similar to FIG. 4A, showinganother embodiment of the cavity flow inhibiting means; and

FIG. 11 is a section of a piping arrangement illustrating a closedbranch pipe which is connected to a main pipe.

THE MOST PREFERRED EMBODIMENT

A various preferred embodiments of the invention will be described belowwith reference to the accompanying drawings.

With reference to FIG. 11, a piping system to which the presentinvention is applied is shown. In FIG. 11, a branch pipe 10 is connectedto a main pipe 22, providing a water main line (hereinafter, referred tomain pipe 22), at a junction 22 a. The branch pipe 10 has a cross pipe12 and a horizontal pipe 14 connected to the cross pipe 12 by an elbowjoint 14, and forms a closed branch pipe when a valve 20, provided in anextended portion 18 of the horizontal pipe 14, is closed. The extendedportion 18 is not limited to the horizontal configuration.

In a large plant such as an electric power plant, a number of branchpipes are connected to a main pipe. Some of the branch pipes are usedduring only maintenance or the starting operation of the plant, and arenot used during the normal operation of the plant with the valves on thebranch pipes closed after the plant is started. A high temperature wateroften flows through such a main pipe. For example, a hot water of about200 Celsius degrees or higher flows through a boiler water supply pipein a conventional electric power plant and a water of about 300 Celsiusdegrees or higher flows through a primary cooling system in a nuclearpower plant.

The branch pipe 10 connected to the main pipe 22 becomes a closed branchpipe when the valve 20 is closed. In the prior art, the flow in such aclosed branch pipe is not considered in the piping design. In the priorart, it has been assumed that because a thermal medium or the water inthe main pipe 22 does not flow into the closed branch pipe 10, thetemperatures of the closed branch pipe 10 and the water thereingradually reduce and the temperature of the pipe reduces, in thedirection from the junction toward the distal end thereof, from thetemperature of the junction 22 a, connected to the main pipe 22, to theambient temperature due to the thermal dissipation through the wall ofthe closed branch pipe 10 after the start of the plant.

With reference to FIGS. 1-3, a laboratory equipment used forvisualization experiment of a cavity flow executed by the inventors willbe described below.

In FIG. 3, a visualization apparatus 100 includes a main pipe 110, abranch pipe 120, a pump for driving a water of a predeterminedtemperature through the main pipe 112 and a tank 104 for holding thewater of the predetermined temperature. Within the tank 104, an electricheater 106 for maintaining the temperature of the water in the tank 104at the predetermined temperature with an electric power source 108supplying the electric power to the electric heater 106. The main pipeincludes an outlet pipe 110 a, a horizontal pipe 110 c providing anentrance region connected at the downstream of the outlet pipe 110 athrough an elbow joint 110 b, a T-joint 110 d for connecting a branchpipe 120 to the horizontal pipe 110 c and a return pipe 110 e forconnecting the T-joint 110 d and the tank 104. The branch pipe 120includes a vertical pipe 120 a connected to the T-joint 110 d and ahorizontal pipe 120 c connected to the lower end of the cross pipe 120 aby an elbow joint 120 b with the distal end of the horizontal pipe 120 cbeing closed by a blind cover 120 d.

In this connection, in the experiment, a pipe of nominal size of 200Awas used as the main pipe 110 and a pipe of nominal size of 100A wasused as the closed branch pipe 120. The horizontal pipe 110 c, providingthe entrance region of the main pipe 110, has 10 m length to eliminatethe influence of flow of the pump 102 and the elbow joint 110 b. Thelength of the horizontal pipe 120 c of the closed branch pipe 120 is2300 mm. Further, the length of the cross pipe 120 a (including thevertically branched portion of the T-joint 110 d) is defined by L1=11.3d(d is the internal diameter of the horizontal pipe 110 c). On the otherhand, the flow within the return pipe 110 e has little influence to theflow in the closed branch pipe 120, and therefore, the arrangementthereof is not limited. A core 112 is disposed in the T-joint 110 d ofthe main pipe 110. By changing the size of the core 112, the flowvelocity of the water through the T-joint 110 d is changed.

Further, in the experiment, case (i), the normal temperatures water, awater of around 20 Celsius degrees, was used for the flows in both thewater in the main pipe 110 and the closed branch pipe 120, and case(ii), a hot water, heated to 60-70 Celsius degrees by the electricheater 106 in the tank 104, was used for the flow through the main pipe110 and the normal temperature water was used in the closed branch pipe120, were compared. In this connection, the flow in the closed branchpipe 120 was observed by using ink, bubble and polystyrene particleswith the relative density being previously adjusted. Further, in orderto observe the decay in the rotating velocity of the swelling flowgenerated in the closed branch pipe 120, a hot film was disposed at alocation from the wall surface to measure the velocity of the downflow.

The observation results of the flow, when the normal temperature wateris used (case (i)), were shown in FIGS. 1 and 2. In the upper end regionin the closed branch pipe 120 or the region within the branch pipe 120adjacent the junction to the main pipe 110, indicated by referencesymbol “I” in FIGS. 1 and 2, a cellular vortex, vortex fluctuatingstrongly as a two-dimensional cavity flow, is induced by the flowthrough the main pipe 110. With reference, in particular, to FIG. 1B,which is a section along line b—b in FIG. 1A, in the region I, a pair ofleft and right, relative to the main stream in the main pipe 110,vortexes are formed, which develop to the cellular vortex. The cellularvortex has a shape in the form of a hair pin including a downflow in thedownstream side region 122 relative to the flow direction in the mainpipe 110 and a upflow in the upstream side region 124.

In region II under the region I, the cellular vortex become unstable andunclear, and develop to a twister vortex described below. Thus, it isassumed that the region II provides a transition zone from the region Iwhere the clear cellular vortex appears and to the region III where thetwister vortex appears. A twister vortex, which includes a rotatingdownflow along the pipe wall and a upflow at the central region of thepipe, appears in the region III under the region II. The reason for thedevelopment of the above-described cellular vortex to the twister vortexis assumed that the cellular vortex cannot exist stable in the region IIbecause of the circular cross-sectional shape of the branch pipe 120.

As described above, clear cellular vortex is appears in the region I.The cellular vortex has strong flow components in the axial direction ofthe closed branch pipe 120. The cellular vortex in the region II isunstable and unclear but also have strong flow components in the axialdirection. In the experiment (i) shown in FIGS. 1 and 2, it was observedthat the twister vortex in the region III extends deeply into thehorizontal pipe 120 c beyond the elbow joint 120 b. The twister vortexis a helical vortex having strong circumferential flow components andweak axial flow components. In the case of experiment (i), it wasobserved that the rotating flow disappears at the distal end of thetwister vortex and weakly layered natural circulation is generated. Thereason for the generation of the natural circulation is assumed that thetemperature of the water flowing through the main pipe 110 becomesslightly higher than the normal temperature of the water in the closedbranch pipe 120 due to the heat input from the pump 102.

On the other hand, in the case of the experiment (ii), when thetemperature difference between the waters in the main pipe 110 and inthe closed branch pipe 120 is about 40 Celsius degrees, a stable thermalstratification appears in the middle of the elbow joint 120 b and theabove-described natural circulation is inhibited. In particular, theboundary surface of the thermal stratification appears in the elbowjoint 120 b at level T1, substantially the same as the top of thehorizontal pipe 110 c. The reason for this is assumed that the axialflow components of the twister vortex is gradually decayed and the heatdissipation through the horizontal pipe 110 c increases toward thedistal end thereof so that the stability of the thermal stratificationis increased due to the turbulence inhibitory action by the buoyantforce.

Thus, it is assumed that the cavity flow induced within the closedbranch pipe 120 is extended to the end of the twister vortex, and thelength of the cavity flow in the closed branch pipe 120 is defined bythe decay characteristics of the axial components of the twister vortexand the stability of the thermal stratification.

In this connection, the twister vortex did not appear within a regionfrom the opening 114 to the range of about six times of the innerdiameter “d” of the closed branch pipe 120. Thus, the terminal end ofthe transition zone II was observed at a position farther than the rangeof six times of the inner diameter “d” of the closed branch pipe 120.This is because the cellular vortex has an extent about three times ofthe inner diameter of a pipe and the region II is a zone where thesecond cellular vortex is generated.

When the thermal stratification is generated, in the proximal regionfrom the thermal stratification T1 to the opening 114, the water in themain pipe 110 circulates in so that the temperature thereof issubstantially the same as that of the water in the main pipe 110. In thedistal region from the thermal stratification T1 , the water in the mainpipe 110 does not circulate in so that the temperature is maintained tothe initial temperature, i.e., about 20 Celsius degrees so that thetemperature suddenly changes across the thermal stratification T1 and asteep temperature gradient is generated. This makes a large thermalstress in the pipe around the thermal stratification T1. As describedabove, when a large temperature difference between the water in the mainpipe 110 and that in the closed branch pipe 120 is generated, thethermal stratification T1 appears in the elbow joint 120 b at the levelthe same as the top of the horizontal pipe 110 c. The elbow joint 120 bis a member which has tendency to be broken when a thermal stress isapplied. Therefore, the condition in which a thermal stratificationappears in an elbow joint of a closed branch pipe in a plant for a longterm, it is expected that the elbow joint may be broken. Therefore, itis important to prevent the generation of a thermal stratification in anelbow joint due to the cavity flow induced in such a closed branch pipe.

A thermal stratification is not generated or is generated in the closedbranch pipe near the opening 114, if the water in the main pipe isprevented from circulating in the deep location in the closed branchpipe by preventing or inhibiting the generation of a cavity flow, sincethe thermal stratification is generated in the deep location in theclosed branch pipe, as described above, by the circulation of the waterin the main pipe into the closed branch pipe due to the cavity flow, asone of the measures for preventing the elbow joint 120 b from beingaffected by a large thermal stress.

With reference to FIGS. 4-10, various embodiments of cavity flowinhibiting means will be described below.

In the embodiment of FIG. 4, a swirl preventing plate 30 is provided asthe cavity flow inhibiting means. The swirl preventing plate 30comprises two plate members 30 a and 30 b which extend in planesincluding the axis of the cross pipe 12. The plate members 30 a and 30 bintersect, preferably perpendicularly at the center of the closed branchpipe 120 a as shown in FIG. 4E, to each other with an intersecting lineextending in the flow direction. The swirl preventing plate 30 ispreferably disposed in the transition region II or at a location in theregion III adjacent the transition region II where the twister vortexmay be generated. The swirl preventing plate 30 divides the inside ofthe cross pipe 12 into four volumes. Therefore, the generation of atwister vortex is prevented so that a cavity flow cannot flow beyond theswirl preventing plate 30.

In the embodiment of FIG. 5, a sleeve 32 with a bottom is provided asthe cavity flow inhibiting means. The sleeve 32 comprises a peripheralwall 32 a having an inner diameter larger than the outer diameter of thecross pipe 12 and a bottom 32 b which is connected between theperipheral wall 32 a and the cross pipe 12. The sleeve is disposed so asto enclose the opening 22 a or the junction connected to the main pipe22. Thus, the provision of the sleeve 32 around the opening 22 a of thecross pipe 12 to the main pipe 22 allows the water within a volume 32between the cross pipe 120 a and the peripheral wall 32 a to move by theshear action of the water flowing through the main pipe 22 so thatturbulence is generated around the opening 22 a. This weakens thecellular vortex generated in the cross pipe 12 to prevent the cavityflow from entering into the closed branch pipe 10.

In the embodiment of FIG. 6, a scoop or deflecting member 36 is providedover the opening 22 a as the cavity flow inhibiting means. The scoop orthe deflecting member 36 has preferably a shape of a portion of a sphereand prevents the cavity flow from entering into the closed branch pipe10 by reducing the shearing action of the water flowing through the mainpipe 22 for the water in the closed branch pipe 10.

In the embodiment of FIG. 7A, an orifice 38 is provided as the cavityflow inhibiting means. The orifice 38 reduces the entrance of thecellular vortex and the twister vortex. Thus, the orifice 38 inhibitsthe formation of the cellular vortex in the form of a hear pin. Theorifice 38 may be formed by a central opening 38 b defined by an annularplate member 38 a, as shown in FIG. 7B. The orifice may include a risingportion or collar 38 e provided along the periphery of the centralopening 38 d of the annular plate member 38 c.

In the embodiment of FIG. 8, a tube member 40 having different diametersis provided as the cavity flow inhibiting means. The tube memberincludes a peripheral wall 40 a, which has an inner diameter larger thanthe inner diameter of the closed branch pipe 10 and is connected to themain pipe 22, and an annular bottom portion 40 b which is connected tothe peripheral wall 40 a and the cross pipe 12. Thus, provision of thetube member 40 allows the flow, in the form of a hear pin induced by theflow through the main pipe 22, to impinge against the bottom portion 40b of the tube member 40 and to be broken. Therefore, it cannot enter thecross pipe 12 of the closed branch pipe 10 so that the cellular vortexis weakened.

In the embodiment shown in FIG. 9, entrance radius enlarged portions 42and 44 are provided as the cavity flow inhibiting means. The entranceradius enlarged portions 42 and 44 are tubular members with thesectional area of their flow channels being gradually reduced from themain pipe 22 toward the cross pipe 12. In the embodiment of FIG. 9A, inparticular, the enlarged radius entrance portion 42 has a symmetricconfiguration relative to the axis of the cross pipe 12. In theembodiment of FIG. 9B, the enlarged radius entrance portion 44 has anasymmetric configuration, in which the upstream side in the flowdirection in the main pipe 22 relative to the axis of the cross pipe 12,perpendicularly intersects the main pipe 22 but on the downstream sidethe sectional area of the flow channel is gradually reduced. Thus, theprovision of the entrance radius enlarged portion 42 and 44 between themain pipe 22 and the cross pipe 12 weakens the cellular vortex,therefore, the twister vortex and the cavity flow in the cross pipe 12are weakened.

In the embodiment shown in FIG. 10, the closed branch pipe 10 includesan inclined connecting pipe 46, as the cavity flow inhibiting means, forobliquely connecting the cross pipe 12 to the main pipe 22. In theembodiment shown in FIG. 10, the cross pipe 12 is connected to theinclined connecting pipe 46 by a bend joint having 45 degrees and theinclined connecting pipe 46 is connected to the main pipe 22 toward thedownstream of the flow in the main pipe 22 with angle of 45 degrees.Thus, provision of the inclined connecting pipe 46 between the crosspipe 12 and the main pipe 22 weakens the cellular vortex, therefore, thetwister vortex and the cavity flow in the cross pipe 12 are weakened.

In the embodiment described above, the generation of the cavity flow isprevented or inhibited to prevent the water in the main pipe fromcirculating deeply into the closed branch pipe whereby the formation ofthe thermal stratification in the closed branch pipe 10 or adjacent theopening 22 a is prevented so as to prevent a large thermal stress in theelbow joint 14. In other word, in the embodiment of FIGS. 4-10, theformation of the thermal stratification is prevented or the thermalstratification is formed upstream of the elbow joint 14 on the otherhand, if the thermal stratification is formed in the closed branch pipe10 downstream of the elbow joint 14, the large thermal stress in theelbow joint 14 can be prevented.

As described above, the cellular vortex has strong flow components inthe axial direction, which provide the water in the main pipe 22 withdriving force for circulation into the closed branch pipe 10. If theaxial flow components are large in the closed branch pipe 10, thestratification by buoyant force is avoided or reduced. Therefore, thethermal stratification does not appear in an area where the cellularvortexes exist. On the other hand, the axial flow components of thetwister vortex are weak so that a thermal stratification is easilyappears. Further, in the closed branch pipe 10, region I where a clearcellular vortex appears and region II where a unclear cellular vortexappears, the region II providing a transition zone from the cellularvortexes to the twister vortex are formed. Therefore, by disposing theelbow joint 14 between the regions I and II, the formation of thethermal stratification in the elbow joint 14 can be prevented. Inparticular, a cellular vortex has a size of three times of the innerdiameter of a pipe. Therefore, by disposing the elbow joint 14 within arange within six times of the inner diameter from the opening 22 a, theformation of the thermal stratification in the elbow joint can beprevented.

Incidentally, although the cross pipe 12 is shown to vertically extendin the above-described embodiments, the invention is not limited to thisconfiguration and the cross pipe 12 may be extend vertically,horizontally or an angle therebetween.

Further, although it is described that the main pipe provides a watermain line in a plant in the above-described embodiment, the invention isnot limited to this configuration and any water pipe through which a hotwater above 40 Celsius degrees flow at a relatively high flow rate.

What is claimed is:
 1. A piping system comprising: a main pipe whichallows a water flow; a branch pipe connected to the main pipe; and aswirl preventing plate positioned in the branch pipe at a point which isat least two point three times an inner diameter of the branch pipefarther from a junction portion of the main pipe, the swirl preventingplate including at least two plates intersecting each other along thebranch pipe.
 2. A piping system comprising: a main pipe which allows awater flow; a branch pipe connected to the main pipe; and an annularplate positioned to prevent cavity flow in the branch pipe and having acentral opening and a collar provided along the periphery of the centralopening of the annular plate member.
 3. A piping system comprising: amain pipe which allows a water flow; a branch pipe connected to the mainpipe; and cavity flow inhibiting means for inhibiting cavity flow in thebranch pipe, the cavity flow inhibiting means being positioned in thebranch pipe at a point which is at least two point three times an innerdiameter of the branch pipe farther from a junction with the main pipe.